Increased activity in the communications industry has caused an increase in the use of fiber optic cables used for transmission of information. Right of way for new fiber optic cables, however, is difficult to obtain. In many areas, such as mountainous or rough or rocky terrain, it is very difficult or impossible to bury cable under ground. In these cases, it is advantageous to install fiber optic cables in the air, along existing right of ways occupied by utility and power lines strung along transmission towers or light poles.
Aerial cable installations require a very strong cable suitable to withstand the stresses imposed by long spans between towers and windy or icy conditions. Fiber optic cables are not known for their tensile and shear strength, and by themselves are not suitable for aerial installations. It is well known, however, to thread or weave fiber optic cable inside a heavier cable for support. One well known means of adapting fiber optic cable for use in an aerial installation is to use an aluminum or stainless steel tube as a protective conduit surrounding the optical fibers. The tube is then disposed inside the heavy, aluminum-clad steel strands that are strung between transmission towers as ground wires. These ground wires are provided, as an addition to the power lines, to protect the system from lightning strikes and to lessen the chance of high voltages seeking random grounds in the event of failure. The light transmission along the fibers inside the tube is not affected by electrical surges from lightning or otherwise present in the ground wires. The combination of fiber optic cable and heavy ground wire is well known and used throughout the communications industry as Optical Ground Wire ("OPGW") fiber optic cable.
Along with the increased demand for OPGW cable, the needs for splicing successive lengths of OPGW and accessing the internal fibers at various points along the path of transmission have also increased. The inherent differences in the physical characteristics of the ground wire portion and the fiber optic portion of OPGW, along with their different functions, requires different approaches to the way each portion is spliced or connected to a successive length. First, the ground wire portion is rarely intentionally broken because a discontinuity in a ground wire is undesirable. Fiber optic cable, however, often requires breaking and splicing for maintenance or to install dielectric fiber optic cables used for local access and distribution to the system. Second, splicing ground wires is accomplished merely by bringing the wires into electrical contact and securing them together by a clamp. Fiber optic cables, however, must be properly aligned end to end to insure continuity in the signal transmitted through them. Third, a ground wire splice can slip slightly or be twisted after installation without breaking electrical contact. Spliced fiber optic cables must not move relative to each other or their continuity may be broken.
The most important consideration is that the ground wire portion of OPGW is extremely heavy and bulky relative to the light and delicate fiber optic portion. The two portions, however, are not separated along the path of transmission until a splicing point is reached. Therefore, because the heavy ground wire portion is in close proximity to the fiber splice, it may pull at or strain the fiber cable splice, possibly rendering it unusable. Adequate means must therefore be provided to isolate the ground wire portion, and secure it so its weight and bulk will not be transmitted to the fiber splice.
The communications industry uses specialized devices known as splice closures for splicing lengths of OPGW. A well known embodiment of splice closure is cylindrical in form with an end plate sealing each end of the cylinder. A clamp is provided to secure the OPGW and act as a strain relief against the weight of the cable. Some splice closures require a separate clamp as a strain relief, while others are designed with one or more clamps attached to an end plate.
Splices and access points are generally provided at transmission towers. In a typical installation, the end of the first length of OPGW to be spliced is attached to the transmission tower, with an excess portion allowed to hang down from the point of attachment. The end of the next length of OPGW is also attached to the tower, with an excess portion also allowed to hang down from the point of attachment. A jumper cable is provided between the attachment points to maintain electrical continuity for the ground wires only. The ends of the two excess portions hanging from the tower are brought together and prepared for splicing by stripping away a portion of the ground wire and exposing the fiber optic portion. The ground wire portions are then clamped to some form of strain relief, and the fiber portions are spliced to themselves and to any local-access dielectric fiber optic cables that may be required in the specific installation. The fiber optic splices are then contained within the splice closure, protected from the elements. The assembly is then secured to the transmission tower. To access the splice, the splice closure is removed from the tower, and may be lowered to the base of the transmission tower if the excess portions of OPGW are long enough.
Because the typical installation method is to use enough excess OPGW to allow the assembly to be lowered to the base of the tower, a substantial weight of OPGW may be present at the splice. This means the strain relief clamping means is very important. Previous means of clamping the ground wire portion of the OPGW and relieving the strain created by its weight have not proved satisfactory. One well known embodiment of strain relief is a circle clamp fitted around the cylindrical splice closure, with means provided for securing the OPGW to the circle clamp. The circle clamp, which must be of sufficient strength to withstand the weight of the OPGW, tends to deform the enclosure, especially as the OPGW is subjected to windy conditions, or as the assembly is lowered to the ground for maintenance. Another well known embodiment of splice closure has the strain relief clamps built into the end plate of the enclosure. This unnecessarily adds to the mass and size of the enclosure, as the enclosure must be built to withstand the strain of the heavy cable instead of merely providing a secure housing for the fragile fiber connections. Further, because the strain from the weight of the OPGW is directly transmitted to the enclosure, the enclosure may deform over time.
The embodiments used in the industry often require the entire assembly to be disassembled during routine maintenance and service changes because the strain relief clamps are an integral part of the enclosure. Opening the enclosure requires releasing all or some of these clamps. Adding a fiber to the assembly in these cases also means breaking and re-splicing all the fibers in the enclosure, even though the new fiber to be added is often local-access dielectric fiber optic cable without a heavy ground wire jacket.
What is needed is a means for separating the strain generated by the weight of OPGW from the point where the fibers are spliced, and a means for allowing additional fibers to be added to the assembly, or existing fibers to be removed without disturbing the previously assembled portions, or any of the strain relief clamps.